1. Field of the Invention
This invention pertains to the art of transport refrigeration systems of the type adapted to selectively provide either cooling or heating operations, and in particular to a refrigeration system arrangement in which overloading of the prime mover is prevented during heating and defrost operations.
2. Description of the Prior Art
In one type of typical transport refrigeration unit currently on the market, and which is used for both cooling or heating depending upon the interior box temperature to be maintained, the refrigerant expansion valve in the line to the coil providing heat exchange within the box is of the pressure limiting type so as to limit the suction pressure in the cooling mode of operation. This expansion valve is effectively bypassed in a heating or defrost mode of operation so that it cannot limit the suction pressure. Accordingly, a throttling valve, sometimes also called a suction hold-back valve or a crankcase pressure regulating valve, is provided in the suction line to the compressor to regulate the amount of refrigerant returning to the compressor. By controlling the amount of the returning refrigerant, the load can be controlled in the heating and defrost modes of operation so that the prime mover for the compressor is not overloaded under adverse conditions. However, these valves cause a pressure drop in the suction line, which is detrimental to the system capacity when operating in the cooling mode, when the presence of the throttling valve is not required.
It is an aim of this invention to eliminate the throttling valve while providing means to prevent an overload of the prime mover during any of the different modes of operation of the system.